The present invention relates generally to string DACs (digital to analog converters), and more particularly to string DAC architectures having a reduced number of resistors and switches, reduced output impedance, and reduced output impedance range.
Typically, an N-bit resistor string DAC includes 2N resistors and from 2N or more switches, depending on the complexity of the decoder. Thus, a 10-bit string resistor DAC would include 1024 resistors and at least 1024 switches which require a large amount of integrated circuit area. A large amount of digital decode circuitry for controlling the large number of switches also is required. The integrated circuit chip area increases rapidly with the number of bits. Furthermore, the string resistor DAC speed is reduced by parasitic capacitances associated with the large number of switches.
There are various references that deal with ways to reduce the number of switches in a string DAC. A reference representative of the closest prior art is believed to be commonly owned U.S. Pat. No. 5,808,576 “Resistor-String Digital-to-Analog Converter” issued Sep. 15, 1998 to Chloupek et al. This patent discloses a digital to analog converter including a first array of resistors connected in series, a switch matrix coupled to the first array, a first variable resistor coupled to a first end of the first array of resistors, and a second variable resistor coupled to a second end of the first array of resistors. The first variable resistor and the second variable resistor have a combined resistance that has a fixed value.
FIG. 1 shows a typical 10-bit string DAC 1 including a resistor string 2 having 1024 series-connected identical resistors R0,1 . . . 1023, 1024 switches SW0,1 . . . 1023, and a digital decoder 4 which decodes the 10 digital inputs D0,1 . . . 9. Decoder 4 produces signals on control lines 5-0,1 . . . 1023 which are connected to control terminals of switches SW0,1 . . . 1023 to select one of the 1024 node voltages on conductors 6-0,1,2 . . . 1023. One terminal of each of switches SW0,1 . . . 1023 is connected to one of conductors 6-0,1, . . . 1023, respectively, and the other terminal of each of switches SW0,1 . . . 1023 is connected to conductor 7, on which Vout is produced. Switches SW0,1 . . . 1023 can be N-channel transistors, or they can be CMOS transmission gates, in which case each of the control lines 5-0,1 . . . 1023 includes two conductors conducting logical complement control signals to the N-channel transistor and the P-channel transistor, respectively, which comprise each transmission gate.
For use in conjunction with switched capacitor circuits, it is desirable that a DAC having a differential output signal present the same output impedance on both output conductors. The terminal to which Vin is applied and the terminal which in FIG. 1 is illustrated as being a ground conductor can be differential input terminals of DAC 1. For example, a differential input voltage Vin=Vin+−Vin− can be applied to DAC 1 wherein Vin+ is applied to the upper terminal of resistor R1023 and input signal Vin− is applied to conductor 6-0.
A drawback of string DAC 1 of FIG. 1 is that it requires such large numbers of switches and series-connected string resistors, i.e., 1024 switches and 1024 resistive segments or string resistors. Furthermore, an undesirably large amount of digital decode circuitry is required. Therefore, the amount of required integrated circuit area is relatively large, resulting in high integrated circuit cost for string resistor DAC 1. Another drawback of conventional string resistor DAC 1 is that it has a large magnitude output impedance, the value of which varies over a wide range with respect to the DAC input code D0,1 . . . 9. This is a serious problem in many applications, because that causes settling times of associated switched capacitor circuits to also be dependent on the DAC input code.
Thus, there is an unmet need for a string resistor DAC having a substantially reduced number of resistors and switches.
There also is an unmet need for a string resistor DAC having reduced output impedance.
There also is an unmet need for a string resistor DAC having a reduced output impedance range.
There also is an unmet need for a string resistor DAC in which the output impedance is relatively invariant with respect to the value of the digital input number.
There also is an unmet need for a string resistor DAC which provides relatively consistent settling times for voltages on sampling capacitors which sample the output of the string resistor DAC.